1. Field of the Invention
This invention relates to a secondary battery having a negative electrode made of a hydrogen storing alloy and also to a method of manufacturing such a secondary battery. More particularly, the present invention relates to an alkaline secondary battery adapted to prevent oxidation/degradation of the hydrogen storing alloy of the negative electrode from occurring during charging where oxygen gas can be generated from the positive electrode; such an adaptation consequently improves the cycle service life and prevents any possible reduction in the capacity of the battery that can appear as a result of prolonged overcharging. It also relates to a method of manufacturing such an improved battery.
2. Related Background Art
In recent years, more research efforts have been directed than ever to realizing improved secondary batteries in light of environmental protection concerns meeting the demand for secondary batteries to be used for proliferated use in portable equipment. As for the issue of environmental protection, attempts have been made to eliminate the emission of carbon dioxide gas from automobiles running on fossil fuel by using electromobiles. The use of secondary batteries has also been considered for the load leveling purposes from the view point of effectively utilizing electric power. Requirements to be met by secondary batteries to be used for such applications include a large capacity, a light weight, a long cycle service life and low cost. On the other hand, secondary batteries to be used for portable equipment are especially required to have a large capacity and to be compact and lightweight. Nickel/hydrate secondary batteries and lithium secondary batteries have been made commercially available as they can meet the above requirements. As a matter of fact, massive R&D efforts are currently being concentrated on such secondary batteries.
The idea of producing nickel/hydrate secondary batteries using a negative electrode made of a hydrogen storing alloy is based on the use of LaNi.sub.5. However, the use of LaNi.sub.5 is accompanied by a number of problems including the tendency of generating hydrogen in the charging stage due to a high equilibrium pressure of hydrogen occlusion, reducing the electroconductivity as LaNi.sub.5 expands and becomes finely powdery in the charging stage and corroding to make La react with alkali and produce La(OH).sub.3. These problems made the nickel/hydrate secondary battery using LaNi.sub.5 commercially not unfeasible in the early days of development.
Later on, a method for replacing part of the La or part of the Ni with some other element, while maintaining the crystal form (AB.sub.5 structure) of LaNi.sub.5 was invented. For example, it is now possible to suppress the expansion of the alloy that can take place during the charging operation and reduce the cost of such a secondary battery by replacing part or all of the La with Mm (misch metal: roughly refined lanthanoid such as La, Ce, Pr or Nd). It is also possible to reduce the equilibrium pressure of hydrogen occlusion by replacing part of the Ni with Al and improve the corrosion resistance by partly replacing with Co. Again, it is possible to suppress the tendency of the alloy of becoming finely powdery by partly replacing with Mn. In fact, negative electrodes using a hydrogen storing alloy have been made commercially feasible by an appropriately combined use of such replacement elements.
Meanwhile, it has been found that hydrogen storing alloys such as ZrMn.sub.2 (AB.sub.2 structure), TiNi (AB structure), and Mg.sub.2 Ni (A.sub.2 B structure) having a crystal form other than that of the AB.sub.5 structure can also be used for the negative electrode of an alkaline secondary battery. R&D efforts have also been directed to this field of technology. As a matter of fact, such hydrogen storing alloys are found to occlude more hydrogen than alloys with the AB.sub.5 structure and hence are more promising when used as negative electrodes. Additionally, one or more than one of the metals of such hydrogen storing alloys can be replaced in a manner as described above by referring to metals having the AB.sub.5 structure if they are used for the negative electrode. For example, Zr may be partly replaced by Ti, and Mn may be partly replaced by Ni, Co, Cr, Al or Fe for the purpose of preparing a commercially feasible negative electrode.
However, if compared with conventional nickel--cadmium secondary batteries, nickel/hydrate secondary batteries have a drawback in that the hydrogen storing alloy of the negative electrode is apt to be oxidized when the nickel electrode generates oxygen when an overcharged. More specifically, the hydrogen storing alloy is highly apt to be oxidized to reduce the capacity of the battery when the charge/discharge cycle is repeated or when a long charging process continues.
In an attempt to eliminate this drawback, particularly for hydrogen storing alloys of the AB.sub.5 type, Japanese Patent Application Laid-Open No. 5-135763 proposes a method of rapidly cooling a hydrogen storing alloy with a particular composition. Another attempt is to specifically define the composition.
Known proposed methods for improving the performance of a hydrogen storing alloy that may or may not show the AB.sub.5 structure include those described below.
Firstly, Japanese Patent Applications Laid-Open Nos. 8-250099 and 9-45331 disclose the use of a water-repelling layer on the surface of the negative electrode or of the separator at the side of the negative electrode to improve the oxygen gas absorbing performance and prevent oxidation/degradation of the hydrogen storing alloy. An oxygen gas absorbing reaction takes place on the surface of the hydrogen storing alloy as oxygen chemically reacts with hydrogen occluded in the hydrogen storing alloy and as oxygen is electro-chemically reduced to produce hydroxy ions on the surface. It is said that electro-chemical reduction also occurs on the surface of nickel that operates as an electricity collector. An oxygen gas absorbing reaction is observed particularly on the interface of the three layers of hydrogen storing alloy, alkaline electrolyte and oxygen gas ,and the use of a water-repelling layer is believed to increase the area of the interface. Thus, while the use of a water repelling layer is highly effective for reducing the internal pressure of the battery, it causes the hydrogen storing alloy to directly contact oxygen gas to consequently oxidize/degrade the alloy. This technique may be effective when a particular composition showing the AB.sub.5 structure is used.
Secondly, Japanese Patent Application Laid-Open No. 8-138658 discloses a method of adding zinc oxide or magnesium oxide to the hydrogen storing alloy of the battery, and Japanese Patent Application Laid-Open No. 5-41210 discloses a method of adding an oxide or hydroxide of copper or bismuth to the negative electrode made of hydrogen storing alloy. With either of these methods, oxygen gas is consumed by the catalytic effect of the added oxide or hydroxide. The hydrogen storing alloy of the battery may not be degraded by oxygen with either of these methods because oxygen does not directly contact the hydrogen storing alloy. However, oxygen gas would not be absorbed completely and satisfactorily through the catalytic effect of the added oxide or hydroxide that consequently, the internal pressure of the battery will rise considerably.
Thirdly, Japanese Patent Application Laid-Open No. 8-31416 discloses a method of forming a nonelectrolytic nickel plating layer around the hydrogen storing alloy, and Japanese Patent Application Laid-Open No. 6-163072 discloses a method of cobalt plating the surface of the hydrogen storing alloy. With either of these methods, an oxygen gas absorbing reaction takes place on the surface of the cobalt layer or the nickel layer, whichever appropriate, so that the hydrogen storing alloy may be minimally degraded and oxygen gas may be absorbed relatively quickly. While these methods are more advantageous than the above-listed methods as they do not give rise to any significant flaws in the initial stages of assembling batteries, the hydrogen storing alloy is broken into small pieces as the charge/discharge cycle is repeated to produce additional surfaces, which will then gradually be oxidized/degraded.
Finally, there has been proposed a method of suppressing generation of oxygen gas from the positive electrode to keep the negative electrode of the hydrogen storing alloy free from excessive load by controlling the charging process. However, in the case of an alkaline secondary battery, the proposed method reveals a drawback in that a sufficient battery capacity cannot be provided unless the battery is electrically charged to such a potential range that gives rise to the generation of oxygen gas from the positive electrode. Therefore, the proposed method significantly detracts from the remarkable advantage of a high capacity of a nickel/hydrate secondary battery.
Thus, the hydrogen storing alloy can become oxidized in an accelerated fashion to reduce the capacity of the battery after repeating the charge/discharge cycle or as a result of overcharging.